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The composition and crystallinity of the near-surface regions of weathered alkali feldspars

Lee, M. R., Hodson, M. E., Brown, D. J., MacKenzie, M. and Smith, C. L. (2008) The composition and crystallinity of the near-surface regions of weathered alkali feldspars. Geochimica Et Cosmochimica Acta, 72 (20). pp. 4962-4975. ISSN 0016-7037

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To link to this article DOI: 10.1016/j.gca.2008.08.001

Abstract/Summary

Our ability to identify thin non-stoichiometric and amorphous layers beneath mineral surfaces has been tested by undertaking X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) work on alkali feldspars from pH 1 dissolution experiments. The outcomes of this work were used to help interpret XPS and TEM results from alkali feldspars weathered for <10,000 years in soils overlying the Shap Granite (north-west England). The chemistry of effluent solutions indicates that silica-rich layers a few nanometers in thickness formed during the pH I experiments. These layers can be successfully identified by XPS and have lower Al/Si, Na/Si, K/Si and Ca/Si values than the outermost similar to 9 nm of unweathered controls. Development of Al-Si non-stoichiometry is coupled with loss of crystal structure to produce amorphous layers that are identifiable by TEM where >similar to 2.5 nm thick, whereas the crystallinity of albite is retained despite leaching of Na to depths of tens to hundreds on nanometers. Integration of XPS data over the outermost 6-9 nm of naturally weathered Shap feldspars shows that they have stoichiometric Al/Si and K/Si ratios, which is consistent with findings of previous TEM work on the same material that they lack amorphous layers. There is some XPS evidence for loss of K from the outermost couple of nanometers of Shap orthoclase, and the possibility of leaching of Na from albite to greater depths cannot be excluded using the XPS or TEM results. This study demonstrates that the leached layer model, as formulated from laboratory experiments, is inapplicable to the weathering of alkali feldspars within acidic soils, which is an essentially stoichiometric reaction. (C) 2008 Elsevier Ltd. All rights reserved.

Item Type:Article
Divisions:Faculty of Science > School of Archaeology, Geography and Environmental Science > Department of Geography and Environmental Science
Faculty of Science > School of Archaeology, Geography and Environmental Science > Earth Systems Science
Interdisciplinary centres and themes > Soil Research Centre
ID Code:3740
Uncontrolled Keywords:RAY PHOTOELECTRON-SPECTROSCOPY LABRADORITE SURFACES AQUEOUS-SOLUTIONS ACID DISSOLUTION HIGH-RESOLUTION ALBITE RATES MECHANISM PH TEMPERATURE
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